Self-propelled pneumatic tools for making small diameter holes through soil are well known. Such tools are used to form holes for pipes or cables beneath roadways without need for digging a trench across the roadway. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose that enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth. The striker impacts against the front wall (anvil) of the interior of the tool body, causing the tool to move violently forward into the soil. The friction between the outside of the tool body and the surrounding soil tends to hold the tool in place as the striker moves back for another blow, resulting in incremental movement through the soil.
Some pneumatic tools incorporate movable bits or chisels at the tapered nose section of the tool to more easily penetrate hard ground. Although this concentration of force is useful for penetrating obstructions, total tool displacement per impact is reduced. This inefficiency causes slower tool speeds when the tool is not penetrating hard ground conditions. To the extent the movable chisel and bit mounted on it move outwardly from the tool body during impact, there is also a tendency for soil to enter the gap behind the bit between the bit and tool body and cause the chisel to become jammed. Spektor U.S. Pat. No. 5,031,706 describes using a resilient gasket confined under compression between the movable head and chisel adapter as a means of preventing soil from entering behind the chisel.
Some prior movable chisel tools have also relied on elaborate front end structures which are susceptible to breakage and more difficult to assemble and disassemble than tools wherein the anvil of the tool does not move. Another has the disadvantage of delivering direct impacts to the rear end of an anvil that is threadedly secured in a front end opening of the tool body, damaging the threaded connection and/or making it difficult to disengage the anvil when the chisel requires replacement. See, for example, U.S. Pat. No. 4,462,468.
In normal operation, the chisel is spring-biased to a position at which its rear end protrudes beyond the read end or impact surface of the anvil. If the chisel encounters an obstruction and is not moved all the way forward as a result of receiving an impact from the striker, then all of the striker's energy continues to be transferred to the chisel with each repeated forward stroke until the rear end of the chisel is flush with the impact surface of the anvil. The present invention provides a movable chisel of simple but durable design that allows the tool to penetrate hard ground and maintain tool performance better than other movable chisels currently in commercial use.